The invention relates to a hand-held hammer drill comprising a tool spindle for driving in rotation a tool, a hammer action providing a percussion driving action for the tool, a swash plate drive for driving the hammer action, a countershaft for driving, as needed, the tool spindle and the hammer action, as well as a housing. The hub of a swash plate drive is rotatably supported in a cantilevered arrangement by means of a first bearing in the housing and surrounds coaxially the countershaft.
Such a hammer drill is disclosed in U.S. 2004/0003931 A1, for example. The hammer drill disclosed therein comprises a tool spindle for driving in rotation a tool, a hammer action for percussion-driving the tool, a swash plate drive for driving the hammer action, as well as a countershaft for driving the tool spindle and the hammer action, as needed. The tool spindle, the countershaft, the hammer action, and the swash plate drive are arranged inside a gear housing.
The drive action of the swash plate drive produced by the countershaft and the resulting hammer action can be switched on and off as needed. In the switch-off state of the swash plate drive, the hammer action is inoperative so that only the tool spindle produces a rotating drive action of the tool clamped in the chuck; no percussion is provided. With switched-off hammer action, work can be carried out on sensitive materials such as tile or the like; the absence of percussion impulses prevents cracking of the material to be drilled.
For generating a swash plate drive action that can be switched on and off as needed, in the embodiment according to U.S. 2004/0003931 A1 the hub of the swash plate drive is rotatably supported by means of a ball bearing and a needle bearing on the countershaft. The countershaft itself is supported rotatingly in the gear housing. By means of a claw coupling, a rotating connection between the hub of the swash plate drive and the countershaft can be realized, wherein simultaneously the rotary drive and the percussion drive are active. The countershaft and its bearings are subjected to high mechanical loads and deformations caused thereby. In operation, this causes a load-dependent varying axis spacing of the loaded countershaft in comparison to the unloaded state. Input and output gears of the countershaft generate undesirable noise at the gears. Wear on bearings and gears is high. The bearings must therefore be selected to be appropriately strong. This and the opposite arrangement of the bearings requires a large amount of space.
In drilling operation with the hammer action being switched off, the claw coupling is disengaged so that at this location the torque transmission between the countershaft and the hub of the swash plate drive is interrupted. The countershaft should effect only the rotary drive action of the tool spindle while the swash plate drive supported on the countershaft should be standing still because it is not actively driven. In practice, however, it has been observed that the countershaft rotating within the hub of the swash plate drive has the tendency to entrain the hub of the swash plate drive. Tests have shown that micro-movements between the hub and the countershaft after extended operation with hammer action can cause fretting. Fretting and grease between countershaft and swash plate drive cause this entrainment effect. This effect is worsened by the elastic deformations of the countershaft under load. Despite being switched off, the swash plate drive has the tendency to be entrained. The hammer action that is thus accidentally activated can therefore spoil the drilling result in the case of sensitive work or can even cause damage to the material to be drilled.
EP 0 589 337 B1 discloses a hand-held hammer drill with a tool spindle, a rotary drive, and hammer action. The hammer action is driven by means of a swash plate drive that can be rotatingly coupled to a countershaft. The hub of the swash plate drive surrounds coaxially the countershaft and is supported in the gear housing by means of a ball bearing. In the radial direction between the hub of the swash plate drive and the countershaft, there is a further hub of a drive gear so that in the radial direction a multi-bearing arrangement is provided. In order to be able to adjust various switching conditions for the drilling and/or chipping operation, the drive gear of the countershaft as well as the countershaft itself must be axially movable. The arrangement is thus of a complex construction and resilient in the bearing area; play cannot be excluded.